In general, a fuel cell is provided with a membrane electrode assembly (“MEA” hereinafter) having an anode, a cathode and an electrolyte membrane having proton conductivity put therebetween. In a case of a direct methanol fuel cell (“DMFC” hereinafter), methanol/water solution is supplied to the anode as a fuel and air is supplied to the cathode as an oxidant. The DMFC need not be provided with a reformer for extracting hydrogen from the fuel; thereby it is advantageous in view of downsizing thereof.
The electrolyte membrane has a function of exchanging ions between the cathode and the anode and is necessary to be humidified with water. In general, the water for humidifying is either supplied on the anode side or produced from an oxygen reduction reaction on the cathode side.
It is known that the methanol partly permeates the electrolyte membrane from the anode to the cathode and such methanol is called “crossover methanol”. The crossover methanol reacts with oxygen at the cathode and causes reduction in fuel utilization efficiency and a counter electromotive force so that the power generation of the fuel cell is suppressed.
It is important to properly regulate the concentration of methanol inside the anode. An Excessively high concentration leads to a generation of a large amount of the crossover methanol and hence reduction of the power generation. Moreover, in an extreme case, the excessively concentrated methanol may deteriorate the MEA. On the contrary, excessively low concentration leads to shortage of fuel for the power generation.
The water-methanol mixture regulated in a proper concentration in advance may be stored in a fuel tank. However, in this case, a relatively large fuel tank is necessary. Water may be recovered from the water generated at the cathode and admixed with the methanol so as to be a proper concentration. In this case, concentrated methanol may be stored in the fuel tank so that a relatively small capacity of the fuel tank gives a large energy density. However, additional devices for recovering water are necessary. These conventional practices destroy the advantage of DMFC, namely feasibility of downsizing.